Method and apparatus for the measurement of duration of ultrashort pulses of monochromatic light

ABSTRACT

A method of and apparatus for the measurement of duration of ultrashort pulses of monochromatic polarized light, such as laser pulses. The method comprises splitting the polarized light beam into two identical beams; causing the two split beams to travel through the same optical path and to interfere on a photocathode comprising a thin metal layer disposed in vacuo, the energy required to detach the electrons from the layer being greater than the energy of the incident light photons; producing an image of the multiphotonic interaction zone obtained on the photocathode by means of electrons detached from the photocathode by a multiphotonic process in response to photons in coincidence from the two split light pulse therefrom. The apparatus is appropriate for carrying out the method.

last-defined means having a cycle of operation that is synchronized with said periodic indexing displacements and that is effected in the period between dwells.

11. Means according to claim 7, in which shield means surrounds said source and transport mechanism and has inlet and outlet radiation ports respectively communicating with inlet and outlet locationsin said transport mechanism, separate shield gates for selectively opening and closing said ports, and gate-actuating means having a gate opening and closing cycle synchronized with the indexing period between dwells.

12. Means according to claim 11, in which said shield gates are provided at outer and inner ends of each of said ports, thereby defining a radiation lock at each port, said locks being of length accommodating at least one container length, and means interlacing the open-and-close cycles of the two gates for each port, whereby each port is always shielded.

13. Means according to claim 12, in which load transport means aligned for external communication with the inlet port includes periodically operative load-in control means, and in which load-discharge means aligned for external communication with the inlet port includes periodically operative loadout control means, and means interlacing the cycles of said respective control means during the respective open phases of operation of said outer gates.

14. A transport system for the sequential unit progression of plural like elongated rectangular prismatic containers in a two-dimensional matrix of plural vertically spaced adjacent tiers of plural horizontally adjacent containers, the two dimensions of said matrix establishing a vertical plane and the container elongation axes being horizontal and normal to said plane, each longitudinal end of each said container including an axially outwardly projecting lifting lug, the path of movement of containers through said matrix comprising entry and sweep along a horizontal pass at a first elevation, followed by transfer to a second elevation and reverse sweep along a horizontal pass at said second elevation and then transfer to a third elevation, and thereafter recycling the horizontal sweeps at successive different elevations,

said system comprising a frame, first push bar means including two parallel bars spaced substantially the length of a container and guided for horizontal movement in said frame along the horizontal dimension of said matrix at the elevation of one of said tiers, the containers of said one tier being supported at their lugs on the bars of said first push bar means, said first push bar means including a first actuator for the unitary guided horizontal displacement of all containers of said one tier, second push bar means similar to said first push bar means and including two frame-guided bars at the elevation of another of said tiers and supporting at their lugs the containers of said other tier, said second push bar means including a second actuator for the unitary guided horizontal displacement of all containers of said other tier,

elevator means guided for vertical movement in said frame and including actuating means for selectively lifting the containers in both tiers in a cycle of intermittent displacement involving a first direction out of supported relation with said bars and a second direction back into supported relation with said bars,

and means synchronizing the operation of said first and second actuators in a common cycle of opposed-phase intermittent displacements, said last-defined means also synchronizing the respective intermittent displacements of said cycles in interlaced relation.

15. The system of claim 14, in which said first push bar means serves plural odd-numbered tiers and said second push bar means serves plural even-numbered tiers, and in which said elevator means comprises a first elevator at one end of said matrix with container support means at double-tier spacings and having a cycle of vertical movement from oddnumbered tier levels to even-numbered tier levels and return, a second elevator at the other end of said matrix with container support means at double-tier spacings and having a cycle of vertical movement from even-numbered tier levels to odd-numbered tier levels and return, the up and down strokes of said first and second elevators being synchronized.

16. The system of claim 15, in which said elevator means includes a third elevator intermediate said first and second elevators with container support means at adjacent tier levels and having a cycle of vertical movement which is less than the tier-to-tier spacing but which is sufficient in its up" position to support all containers vertically aligned therewith and in its down" position to have released all containers vertically aligned therewith into support relation with said push bar means.

17. The system of claim 15, in which the container support means of one of said elevators includes at one container support level a roll conveyor defining essentially a surface of container support, and drive means for said roll conveyor.

18. The system of claim 17, in which the direction of container movement determined by driving the rolls of said conveyor is essentially horizontal and normal to said vertical plane.

19. The system of claim 15, in which said frame includes, at one of the tier levels served by one of said elevators, a roll conveyor defining essentially a surface of container support, and drive means for said roll conveyor.

20. Means for the substantially uniform production line irradiation of a material to be irradiated, comprising source means including an elongated source of radiation and means for supporting the same with its elongation axis generally horizontal, a plurality of like elongated rectangular prismatic containers essentially transparent to the radiation for containing unit quantities of the material to be irradiated, and an orthogonal two-coordinate transport mechanism supporting plural tiers of plural containers in a relatively closely nested matrix of vertically spaced adjacent tiers of plural horizontally spaced containers in proximity to said source; the two dimensions of said matrix establishing a vertical plane that is generally normal to the elongation axis of said source; the path of movement of containers through said matrix comprising successively reversed generally horizontal passes at each of a succession of different vertical elevations;

said transport mechanism comprising a frame,

said push bar means including two parallel bars spaced substantially the length of a container and guided for horizontal movement in said frame along the horizontal dimension of said matrix at the elevation of one of said tiers, the containers of said one tier being supported at their lugs on the bars of said first push bar means, said first push bar means including a first actuator for the unitary guided horizontal displacement of all containers of said one tier,

second push bar means similar to said first push bar means and including two frame-guided bars at the elevation of another of said tiers and supporting at their lugs the containers of said other tier, said second push bar means including a second actuator for the unitary guided horizontal displacement of all containers of said other tier, the push bars for one tier and said other tier having the same effective stroke length and being respectively at horizontal elevations above and below said source,

elevator means guided for vertical movement in said frame and including actuating means for selectively lifting the containers in both tiers in a cycle of intermittent displacement involving a first direction out of supported relation with said bars and a second direction back into supported relation with said bars,

and means synchronizing the operation of said first and second actuators in a common cycle of opposed'phase intermittent displacements, said last-defined means also synchronizing the respective intermittent displacements of said cycles in interlaced relation;

said elevator means comprising a first elevator at one end of said matrix with container support means at one of said 1 METHOD AND APPARATUS FOR THE MEASUREMENT OF DURATION OF ULTRASHORT PULSES OF MONOCHROMATIC LIGHT picoseconds (101' sec.) and can therefore advantageously be 1 used in connection with lasers. For example, lasers of the switched type may emit ultrashort pulse trainsof a duration varying from 0.4 nanosecond for ahelium-neon gas laser to less than 2 picoseconds for a solid neodymium-doped glass laser. The pulses are separated by the transit time of photons in the laser cavity, equal to 2L/C where L is the length of the cavity and C is the speed of light. This transit time is usually of the order of a few nanoseconds. It may be desired to isolate a single light pulse from a train of such pulses; since very short light pulses as short can be used to improve the accuracy of certain measurements, such as in laser telemetry, or in the study of fundamental physical phenomena such as the interaction of laser radiation with matter. The duration of the ultrashort light pulses then, needs to be determined accurately.

A number of prior methods have been used for this purpose but their use is relatively inconvenient and involves high levels of light energy. Some methods necessitate repeated operations but in other, direct methods measurement is made at a single time with a single pulse; in consequence, the latter methods are the methods of practical importance.

In one direct method, the light beam is directed on a photocathode sensitive at the wavelength of the incident radiation, the electrons emitted by the photocathode are accelerated in a first direction as an electron beam. The beam is scanned in a direction'perpendicular to the direction of acceleration by means of a very powerful magnetic or electrostatic field. The bearn impinges on a fluorescent screen arranged at right angles to the direction of the beam, and the length of the impact of the deflected electron beam on the screen is measured. Scanning differs from the conventional scanning used with cathode-ray tubes in that it requires a deflection field of very high intensity since the light pulses are spatially very short. This method is very effective but requires deflection means which are difficult to operate and the method is therefore rarely used and is still substantially confined to the laboratory.

A second method is based on the phenomenon of fluorescence by organic solutions subjected simultaneously to the action of two photons. A container containing a suitable organic solution and closed by a mirror at one end is placed in the path of the light beam. The mirror reflects the beam, which traverses the solution in the container a second time. The emission of fluorescent light from the organic solution, which is small or negligible under the action of a single photon but is relatively greater when two photons acting simultaneously, is reinforced when the ultrashort pulses in the incident and reflected phenomenon is photographed and it is then merely necessary to measure the length of the zones in which the emission of fluorescent light is reinforced. A rhodamine solution can be used as the organic solution for lasers having a wavelength at l.06p..

In another method, a photographic film or plate which is very insensitive to the effect of a single photon is darkened by the simultaneous action of two photons.

The main disadvantage of the direct methods mentioned is the fact that they are suitable for measurements on very weak ultrashort pulses; the pulses used must each have an energy of at least some hundreds of millijoules. These methods cannot therefore be used to measure the duration of light pulses from a laser generator without some amplifying means. Further, methods involving organic solutions are inconvenient since the solutions exhibit fatigue when exposed to light, and have therefore to be used in the dark, and frequently renewed.

light' beams are in coincidence. The

The invention provides ameasuring method and apparatus which meets practical requirements better than prior art devices in that it is free from the disadvantages mentioned. The invention is directed more particularly to a method for measuring ultrashort light pulses having an energy which may be very low, such as a few millijoules, requiring only a single pulse.

More specifically, suring the duration of a short pulse of substantially monochromatic light, which comprises splitting a polarized beam of the light into two paths causing light. in the two paths, after having travelled through the same optical path, to interfere on the surface of a photocathode comprising a thin metal layer disposed in vacuum of a nature such that the energy required to detach electrons from the layer isgreater than the energy of the incident light photons, obtaining an image of the multiphotonic interaction zone the photocathode by means of electrons detached from the photocathode surface by a multiphoton process by means of photons in coincidence pertaining to light in the two paths and not to a light in a single path, measuring the extent of the said interaction zone and deducting the duration of the light pulse therefrom.

When only a few electrons are detached from the photocathode, that is when measurement isrmade on a light pulse of low energy, the electron current is amplified before being used to produce the image of theinteraction zone.

The invention also provides an apparatus for carrying out the method comprising a beam splitter for splitting the light beam into two beams, a photocathode comprising a thin metal layer disposed in vacuum, means for causing the two identical beams to interfere on the photocathode, and means for forming an image of the multiphotonic interaction zone of the two beams on the cathode, by means, of electrons extracted from the metal layer by a multiphotonic method.

In one form of apparatus, there is used a system for proportionally increasing the number of electrons from the metal layer disposed immediately after the photocathode with means such as a photographic plate or a fluorescent screen responsive to the increased number of electrons.

The invention will be more clearly understood from the following description of an exemplary nonlimitative embodiment. The description refers to the accompanying drawings, in which:

The FIGURE diagrammatically shows a device according to the invention for measuring ultrashort light pulses.

The method makes use of the nonlinear photoelectric effect observed in metals such as gold, cesium, silver and nickel. A photon having an energyh which is less than the energy required to detach an electron from the metal will not give rise to a photoelectric emission, unless a number of such photons act simultaneously. In this specification, and in the appended claims, this phenomenon is referred to as multiphotonic. It is found by experiment that the number n of extracted electrons is given by the equation:

in which k is a constant, E is the electric filed associated with the light wave and a is a coefficient depending upon the nature of the metal forming the photocathode and on the wavelength of the incident photons.

The value of the coefficient a is given by the ratio of the energy required to detach electrons from the metal to the energy hv of the incident photons; a has a value of 2 for cesium and 6 for gold and silver. in the case of a plane wave propagating in a nonmagnetic medium having a refractive index n, the relationship between light energy and the value of the electric field E is given by:

where e" is impedance of vacuum, that is, 377 ohms.

ln carrying out the method, therefore, ultrashort pulses of monochromatic light, in a substantially parallel beam, are split intotwo beams, and light in the two beams is used to causeinthe invention provides a method of meaterference. The beams travel through the same optical path on to a photocathode for which the energy required to detach electrons is greater than the energy of the incident photons. In this way, the splitting of the initial beam is used to produce interference between two pulses which are identical with the initial pulse whose duration is to be measured.

Multiphotonic action occurs between the photons of the coincident beams and the electrons from the metal layer of the photocathode, which consists of a thin metal layer disposed in vacuum. A certain number of the electrons are emitted; if E,(t and E (t) denote the values at an instant t of the electric field associated with the light wave of each of the two beams, the electric field resulting from interference between the two beams at a point on the photocathode has the form E(t)=E (t) +E- g(l+'T), where r is the delay between the two interfering light waves at the point on the'photoca'thode 'i'nquestion.

If the photocathode response is nonlinear, it will contain a term proportional to the correlation function 3(7) of two pulses:

The background noise, other than thermal noise, consists of the multiphotonic response of the photocathode under the action of at least two photons belonging to one of the two split beams. It has been found by experience that the photoelectric emission is reinforced in those regions of the photocathode which are scanned by the pulses in coincidence. The dimensions of the coincidence or multiphotonic interaction zones are directly related to the duration of the ultrashort pulses. When the number of electrons emitted from the metal layer is insufficient to form a direct image of the coincidence zone, they are effectively multiplied by suitable means.

In the FIGURE, which shows an arrangement for carrying out the method, a parallel beam 1 of ultrashort pulses of monochromatic light to be measured is split by a beam splitter 2 consisting of a thin partly reflecting, partly transmissive plate having a reflection coefficient of 0.5, by which two identical beams 3 and 4 are obtained. The beams are reflected by mirrors 5 and 6 and then caused to produce an interference pattern on the surface of a photocathode 7 disposed in a plane perpendicular to the plane determined by the electric field and the direction of propagation along the same angle of incidence 0 and after having travelled along the same optical path from the splitter 2. Incidence approaches the tangential, that is 6, the angle of incidence, is approximately 90. This angle is selected because the electric field E(t) associated with the polarized light of beams 3 and 4 is then a maximum, since the direction of the field E(t) is substantially normal to the surface of photocathode 7. The photocathode comprises a thin metal layer disposed in a secondary vacuum and grounded. Behind photocathode 7 and opposite the interference surface of beams 3 and 4 there is an electron multiplier system consisting of parallel fibers 8 which are polarized by a DC generator 9 to a potential difference of the order of a few kilovolts. Each fiber, which has a diameter of the order of some tens of microns, is hollow and behaves like an electron multiplier having distributed dynodes. Such fibers can be obtained, for example, from ,Laboratoire dElectronique et de Physique Appliquee." The fibers amplify the number of electrons receive in that region of the photocathode where the nonlinear photoelectric effect occurs. The multiplying factor may be very large and may reach 10 After emerging from the fibers 8, the electrons are received on a fluorescent screen 10 which they cause to fluoresce, and this region is photographed to give the width l of the zones of coincidence of the ultrashort pulses. The value of I is given very simply by the relation:

I=CAt/ sin 0 where At is the duration of the ultrashort pulses, C is the speed of propagation of light in the medium above the photocathode, and 0 is the angle of incidence. Since the value of I can be determined by photography, and 0 is known from the geometrical conditions, the value of At, the duration of the ultrashort pulses can be calculated from the relation:

At=l sin B/C The arrangement shown in the FIGURE has numerous advantages. For example, the duration of an ultrashort pulse is measured directly, that is, at a single time and on a single pulse. Secondly, the method of measurement is extremely sensitive since the nonlinear photoelectric effect is observable at energies of only a few millijoules, in view of the high gain (10) of the electron multiplier fibers. The device can be used to measure the duration of low-energy laser signal pulses as short as a few picoseconds 10 seconds).

The invention is not limited to the embodiment which has been shown and described by way of example. More particularly, a number of references have been made to light pulses from a laser generator, but the present method of measurement is equally applicable to light pulses from a noncoherent but substantially monochromatic source. Furthermore, the fluorescent screen 10 can be replaced by a photographic plate whose exposed density can be measured to evaluate the duration of the pulses by measuring the dimensions of the zone at which an image is produced by the electrons. As shown, the position of mirrors 5 and 6 with respect to photocathode 7 is such that the angles of incidence 0 of the two light beams 3 and 4 are the same; this is convenient but not essential. If the angles of incidence are not equal, the formula relating the duration At of the ultrashort light pulse to the width 1 of the multiphotonic interaction zone on the photocathode is slightly more complicated than the formula cited in the case of the device described in the FIGURE. The electron multiplier system is not necessarily a system of fibers; other suitable means can be used.

I claim:

1. A method of measuring the duration of ultrashort pulses of substantially monochromatic polarized light which comprises splitting the polarized light beam into two identical beams; causing the two split beams to travel through the same optical path and to interfere on a photocathode comprising a thin metal layer disposed in vacuo, the energy required to detach electrons from the layer being greater than the energy of the incident light photons; producing an image of the multiphotonic interaction zone obtained on the photocathode by means of electrons detached from the photocathode by a multiphotonic process in response to photons in coincidence from the two split light beams jointly and not to a photon from one beam; measuring the extent 1 of the aforementioned zone and deducing the duration of the light pulse therefrom.

2. A method according to claim 1, and comprising the step of providing that the split beams interfere on the photocathode at the same angle of incidence, and deducing the duration At of the light pulse from the equation:

At=I sin 0/C where 9 is the angle of incidence and C the speed of light in the medium in which the two beams arrive at the photocathode.

3. A method according to claim I, wherein the two split beams are incident on the surface of photocathode at substantially glancing angles of incidence.

4. A method according to claim 1, which comprises increasing the number of electrons detached from the photocathode and using the increased number of electrons to produce the image of the multiphotonic interaction zone.

5. A device for measuring the duration of ultrashort pulses of substantially monochromatic light, comprising means for producing a pulsed beam of said light, beam splitting means for splitting said light beam into two identical beams, an electron-emissive photocathode comprising a thin metal layer disposed in a vacuum, optical path defining means for directing the said identical beams to interfere on the surface of the photocathode, and image forming means for forming an image of the multiphotonic interference zone of the two beams on the said surface cathode from electrons emitted from the metal layer multiphotonically.

6 6, A device according to claim 5, wherein said image fonn- 8. A device according to claim 6, wherein last means is a ing means comprise an assembly of parallel electron-multiplifl or ent screen on which said electrons impinge. fibers, Polarizing means for Polarizing d fibers to a high 9. A device according to claim 6, wherein the fibers have a potential difference said polarizing means being disposed very Small diameter of the order of a few tens of micmrm under the metal layer, and means responding to the multiplied 5 10 A device according to claim 5 wherein the number of electrons.

7. A device according to claim 6, wherein said last means is photocathode electncany connected to ground a photographic plate. 

1. A method of measurinG the duration of ultrashort pulses of substantially monochromatic polarized light which comprises splitting the polarized light beam into two identical beams; causing the two split beams to travel through the same optical path and to interfere on a photocathode comprising a thin metal layer disposed in vacuo, the energy required to detach electrons from the layer being greater than the energy of the incident light photons; producing an image of the multiphotonic interaction zone obtained on the photocathode by means of electrons detached from the photocathode by a multiphotonic process in response to photons in coincidence from the two split light beams jointly and not to a photon from one beam; measuring the extent l of the aforementioned zone and deducing the duration of the light pulse therefrom.
 2. A method according to claim 1, and comprising the step of providing that the split beams interfere on the photocathode at the same angle of incidence, and deducing the duration Delta t of the light pulse from the equation: Delta t l sin theta /C where theta is the angle of incidence and C the speed of light in the medium in which the two beams arrive at the photocathode.
 3. A method according to claim 1, wherein the two split beams are incident on the surface of photocathode at substantially glancing angles of incidence.
 4. A method according to claim 1, which comprises increasing the number of electrons detached from the photocathode and using the increased number of electrons to produce the image of the multiphotonic interaction zone.
 5. A device for measuring the duration of ultrashort pulses of substantially monochromatic light, comprising means for producing a pulsed beam of said light, beam splitting means for splitting said light beam into two identical beams, an electron-emissive photocathode comprising a thin metal layer disposed in a vacuum, optical path defining means for directing the said identical beams to interfere on the surface of the photocathode, and image forming means for forming an image of the multiphotonic interference zone of the two beams on the said surface cathode from electrons emitted from the metal layer multiphotonically.
 6. A device according to claim 5, wherein said image forming means comprise an assembly of parallel electron-multiplier fibers, polarizing means for polarizing said fibers to a high potential difference said polarizing means being disposed under the metal layer, and means responding to the multiplied number of electrons.
 7. A device according to claim 6, wherein said last means is a photographic plate.
 8. A device according to claim 6, wherein last means is a fluorescent screen on which said electrons impinge.
 9. A device according to claim 6, wherein the fibers have a very small diameter of the order of a few tens of microns.
 10. A device according to claim 5, wherein the photocathode is electrically connected to ground. 